You could try taking the cover off the non-working socket, if it's not working you can't make it any worse. Look for a wire that has come adrift, you should have wires going to terminal numbers 2 and 5 (line) and 3 (bell circuit). If all OK, trace the cable back and check for any damage, particularly if you've had new carpets laid, or work on floorboards or skirting. You should eventually come to another socket or a junction box. At this point, if you haven't found a wire adrift or a nail through the cable, then call in a pro. Further probing in the other boxes could make things worse.